> The one way I can think it would get more expensive is that the pool of drivers with insurance would get smaller. That would naturally drive the rates and such up.

Why would it increase the rates?

Rates are mostly driven by statistical analysis taking risk of accidents and cost of repairs into consideration. It is not a product particularly fit for a volume discount. If anything, fewer drivers on the road should mean fewer accidents, if the area/volume of roads stays the same (because of the drop in density, which means a drop in probability to be in the same spot as another driver at a given time).

> The best analogy would be insurance for historical or classic vehicles; if you've ever looked into it, not only do they require certain things normal car insurance doesn't (for instance, they will typically only insure you if your car is kept in an enclosed garage), but because the pool of drivers is much smaller, it is more expensive to get.

I don't think it's due to the pool of drivers being smaller. I think it's because the incident cost is much higher than high volume cars, because both parts and labor are more expensive for low volume productions (custom/exotic). Source: my exotic car.